Recent studies have emphasized that mitochondria in endothelial cells play an important role in signaling in response to environmental cues, including nutrient excess. Mitochondrial signaling is mediated in large part by regulated production of reactive oxygen species (ROS) by components of the electron transport chain. Physiological signaling depends on control of membrane potential by uncoupling protein-2 (UCP2) and preserved integrity of mitochondrial proteins and mtDNA via an appropriate balance between mitochondrial fission and fusion to maintain normal mitochondrial networks (mitochondrial dynamics). Obesity is associated with an imbalance between energy supply and demand in the body. We hypothesize that chronic energy excess creates a vicious cycle of increased ROS that triggers mitochondrial fragmentation and an inadequate UCP-2 response that further increases ROS and impairs endothelial function. In this project, we will relate nitric oxide signaling and endothelium-dependent vasodilation to relevant aspects of mitochondrial function in freshly isolated arterial endothelial cells from obese patients and from healthy volunteers exposed to two human models of energy excess. Our preliminary data show impaired eNOS signaling, decreased UCP2, mitochondrial fragmentation, and an increase in the fission protein Fis1 in endothelial cells collected from obese patients. Our project has 3 specific aims: For Aim 1, we will collect arterial endothelial cells from obese patients and measure mitochondrial ROS, network extent, and expression of UCP-2, Mfn2, and Fis1 and relate the findings to endothelium- dependent vasodilation in the arm and to eNOS activation in the freshly isolated cells. We will also determine whether silencing Fis1 or over-expressing UCP-2 or Mfn2 restores eNOS activation. In Aim 2, we will determine whether altered dynamics and UCP-2 contribute to endothelial dysfunction induced by Intralipid infusion (energy excess), and in Aim 3, we will determine whether these mechanisms contribute to endothelial dysfunction induced by bed rest (decreased energy demand). We anticipate increased ROS, network fragmentation, decreased UCP2, and impaired eNOS activation in cells from obese patients. If Intralipid and bed rest induce an obese endothelial phenotype and if over- expressing UCP-2 or silencing Fis1 reverses endothelial dysfunction, we will have strong evidence that these mechanisms contribute to the pathogenesis of endothelial dysfunction in human obesity.